Lava penetrating water: the different behaviours of pāhoehoe and ‘a‘ā at the Nesjahraun, Tingvellir, Iceland |
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Authors: | John Alexander Stevenson Neil Charles Mitchell Fiona Mochrie Michael Cassidy Harry Pinkerton |
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Institution: | (1) S.E.A.E.S., University of Manchester, Williamson Building, Oxford Road, Manchester, M13 9PL, UK;(2) Lancaster Environment Center, Lancaster University, Lancaster, LA1 4YQ, UK |
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Abstract: | The Nesjahraun is a basaltic lava flow erupted from a subaerial fissure, extending NE along the Tingvellir graben from the
Hengill central volcano that produced pāhoehoe lava followed by ‘a‘ā. The Nesjahraun entered Iceland’s largest lake, Tingvallavatn,
along its southern shore during both phases of the eruption and exemplifies lava flowing into water in a lacustrine environment
in the absence of powerful wave action. This study combines airborne light detection and ranging, sidescan sonar and Chirp
seismic data with field observations to investigate the behaviour of the lava as it entered the water. Pāhoehoe sheet lava
was formed during the early stages of the eruption. Along the shoreline, stacks of thin (5–20 cm thick), vesicular, flows
rest upon and surround low (<5 m) piles of coarse, unconsolidated, variably oxidised spatter. Clefts within the lava run inland
from the lake. These are 2–5 m wide, >2 m deep, ∼50 m long, spaced ∼50 m apart and have sub-horizontal striations on the walls.
They likely represent channels or collapsed tubes along which lava was delivered into the water. A circular rootless cone,
Eldborg, formed when water infiltrated a lava tube. Offshore from the pāhoehoe lavas, the gradient of the flow surface steepens,
suggesting a change in flow regime and the development of a talus ramp. Later, the flow was focused into a channel of ‘a‘ā
lava, ∼200–350 m wide. This split into individual flow lobes 20–50 m wide along the shore. ‘A‘ā clinker is exposed on the
water’s edge, as well as glassy sand and gravel, which has been locally intruded by small (<1 m), irregularly shaped, lava
bodies. The cores of the flow lobes contain coherent, but hackly fractured lava. Mounds consisting predominantly of scoria
lapilli and the large paired half-cone of Grámelur were formed in phreatomagmatic explosions. The ‘a‘ā flow can be identified
underwater over 1 km offshore, and the sidescan data suggest that the flow lobes remained coherent flowing down a gradient
of <10°. The Nesjahraun demonstrates that, even in the absence of ocean waves, phreatomagmatic explosions are ubiquitous and
that pāhoehoe flows are much more likely to break up on entering the water than ‘a‘ā flows, which, with a higher flux and
shallow underlying surface gradient, can penetrate water and remain coherent over distances of at least 1 km. |
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